As disclosed in copending U.S. patent application Ser. No. 220,369 to applicant and others and assigned to the same assignee as the present invention, the curing cycle of thermosetting resin composite systems generally involves the application of heat to initiate and sustain polymerization while volatiles are driven off, followed by the application of pressure to the composite when crosslinking takes place, i.e., at the onset of gelation but before hardening of the composite. The pressure is applied to the composite to compact same, thereby squeezing out excess resin and eliminating voids in the composite.
The exact time of applying pressure has been found to be critical if optimum mechanical properties in the cured composite are to be achieved. If the pressure is applied too early or too late in the cure cycle, the cured composite has an unacceptable number of voids therein and an unacceptably high resin/fiber ratio.
Dielectrometers have heretofore been used to monitor the capacitance (corresponding to chemical changes occuring) and dissipation (viscosity) of a composite during its cure cycle, with such data being used to develop a characteristic wave form for a given type of composite material. The information developed was then used to determine the best time for applying pressure to the composite.
In the case of polymeric materials having addition-type reactions, the capacitance curve is relatively featureless and incapable of defining the optimum time for applying pressure. The dissipation curve does have two fairly definitive maximum levels established on the curve with the second maximum occuring in the general vicinity of a so-called "pressure application window" (which is defined on one side by the onset of gelation and on the other side by the hardening point of the polymer). This second maximum has been used as a basis for triggering the pressure application with some success when operating in the "manual" mode. Some operators have attempted to initiate pressure application at points on the upward slope of the curve just prior to reaching the maximum in an attempt to avail themselves of the entire width of the pressure window, but this has only been met with moderate success.
Accordingly there is a need for a simple, accurate and efficient method for developing a control signal characteristic which is indicative of the optimum time for applying pressure to an addition-type reaction composite in order to obtain optimum mechanical characteristics in a completely cured composite. Such a method should preferably be automatic, have a flexibility capable of handling different types of addition-type reaction resin systems, complete the curing process in a minimum amount of time, and render a cured composite which is substantially free of voids and which has minimal resin content lying in an acceptable range. Further, it would be advantageous if a single computer program could be utilized for the curing of a number of different types of polymers.